Toll-like receptors (TLRs) are key sensors of microbial products and are essential for the development of innate immunity to pathogens. TLR activation induces the expression of hundreds of genes that encode inflammatory cytokines, antimicrobial proteins, and regeneration and metabolic regulators;these molecules in turn mediate inflammation, antimicrobial immunity and tissue regeneration seen in patients with infectious diseases. However, uncontrolled or prolonged activation of TLRs can have devastating consequences, which include the development of septic shock and fatal inflammatory diseases. Fortunately, TLR activation is tightly controlled by two classes of negative regulators: a) signal-specific regulators that inhibit TLR signaling, and b) gene-specific regulators that suppress TLR target gene transcription. These regulators ensure that prolonged or repeated exposure of TLRs to their ligands does not lead to sustained activation of the receptors;instead, it renders them insensitive or hyporesponsive to subsequent ligand stimulation. This phenomenon is referred to as TLR tolerance, or lipopolysaccharide (LPS) tolerance when LPS is the ligand involved. Recent genomic profiling of LPS responses reveals that LPS tolerance is a gene-specific phenomenon, i.e., it selectively targets one set of genes (e.g., inflammatory genes) but not others (e.g., antimicrobial genes);in fact, the expression of antimicrobial genes is further upregulated in LPS tolerized cells. Because it is the inflammatory genes, not the antimicrobial genes, that cause deleterious inflammatory responses, LPS tolerance ensures that the host is able to continuously build up its antimicrobial immunity without causing fatal inflammatory diseases even with chronic or prolonged infections. However, the molecular mechanisms through which prolonged LPS exposure activates antimicrobial genes, but paradoxically suppresses inflammatory genes are unknown. We recently discovered that B cell leukemia (Bcl)-3 mediates LPS tolerance by inhibiting nuclear factor (NF)-kB, one of the three major transcription factors activated by LPS. The goal of this investigation is to elucidate the molecular mechanisms through which Bcl-3 regulates TLR activation and tolerance. Specifically, we will define 1) the roles of NF-kB binding sites of gene promoters in TLR tolerance, 2) the roles of p50 ubiquitination in TLR signaling, and 3) the mechanisms through which Bcl-3 inhibits p50 ubiquitination. Information generated from these studies may not only bring about an important conceptual advance with regard to mechanisms of TLR signaling but also aid in the development of novel Bcl-3-based strategies for the treatment of infectious diseases.